This invention relates to a method and apparatus for nuclear magnetic resonance spectroscopy, and more particularly to a method and apparatus for expanding the decoupling bandwidth.
In nuclear magnetic resonance spectroscopy, spin decoupling technique is used for identifying or simplifying a spectrum split by spin-spin coupling and further for improving sensitivities. Generally, spin decoupling is worked out by putting the unobserved nuclear species coupled with the observed nuclear species in a resonant state, and decoupling the spin-spin coupling, and basically, requires application of a radiofrequency magnetic field having the resonant frequency for the unobserved nuclear species. However, application of a continuous wave radiofrequency field may result in a narrow decoupling bandwidth. Then, for broadening the decoupling bandwidth, a noise modulation method in which the rf field is modulated by random or pseudorandom noise or a square wave phase modulation method in which rf field is phase-modulated by square wave (that is to say, rf field is reversed in phase at a constant period) is used. Nevertheless, neither of the methods can sufficiently broaden the decoupling bandwidth.
FIG. 1 shows the decoupling bandwidth according to each modulation method, where (a) refers to continuous irradiation without modulation; (b) refers to a case of noise modulation with a 1,000 Hz bandwidth; and (c) refers to a case of square wave phase modulation by 100 Hz square wave. For the comparison in FIG. 1, the nuclear species to be decoupled are protons and the resonant frequency (central frequency: 0 Hz off-set) is 100 MHz, and the peak power at (a) is normalized to 1.0.
Assuming that the marginal power at which decoupling is effective is approximately 0.5, it can be shown that the decoupling bandwidth in either case (b) or (c) can ultimately cover only about 1 KHz equivalent to the chemical shift width of protons at 100 MHz.
For broadening the decoupling bandwidth, it is necessary to increase rf power (electric power). There exist limitations upon increasing the rf power due to heating of the sample and the irradiation coil.
In recently developed NMR spectrometers having superconducting magnets, since intensity of static magnetic field is extremely high, the resonant frequency can reach 400 MHz or 500 MHz (each for protons) and it is necessary to expand the decoupling bandwidth correspondingly. The above referenced prior decoupling methods are unsatisfactory for this reason.
More recently, the following pulse decoupling method has been proposed by R. Freeman (Journal of Magnetic Resonance, 43 502, 1981).
As shown in FIG. 2, in this pulse decoupling method, a pulse train which consists of 90.degree. pulses (rf pulses having pulse width for rotating the magnetization by 90.degree.) for the species to be decoupled and 240.degree. pulses (rf pulses having pulse width for rotating the magnetization by 240.degree.) without intervals, is applied repeatedly. In FIG. 2, suffixes x, y, -x and -y indicate the phase of rf carrier in each pulse as follows: x: 0.degree.; y: 90.degree.; -x: 180.degree.; and -y: 270.degree.. In FIG. 3, (d) represents the decoupling bandwidth obtained by the repeated irradiation of this pulse train. It can be found out from the figure that the bandwidth becomes more than several times as broad as those obtained by any of the modulation methods shown in FIG. 1. In addition, since the peak strength also becomes higher than those obtained by the conventional decoupling methods, decoupling can be effectively complete over a wide range and the signal to noise ratio is improved.
This pulse decoupling method has been further advanced in the present invention. Namely, the present inventor has calculated the relations between the off-set frequency and J.sub.R /J.sub.0 in the aforementioned pulse decoupling method under appropriate conditions. (The off-set frequency is the difference in frequency between the resonant frequency of the observed species and the resonant frequency of the species to be decoupled.) J.sub.0 represents a distance between two peaks split by coupling and J.sub.R represents a distance between two peaks reduced by decoupling. The degree of decoupling can be known by the value of J.sub.R /J.sub.0, thereby peaks are completely unified when J.sub.R /J.sub.0 =0 (in other words, a state of complete decoupling).
For instance, calculations can be done as follows. Assuming that I and S represent the spin systems of two nuclear species, and that rf field for decoupling (intensity: H.sub.2, angular frequency .omega..sub.2) is applied to S spins, the spin Hamiltonian is given for a function of time t as follows: ##EQU1## Where, .omega..sub.I, .omega..sub.s are angular frequencies of I and S spin systems; I.sub.X, I.sub.Y, and I.sub.Z are X, Y, Z direction components of the magnetization of the I spin system, S.sub.X, S.sub.Y, S.sub.Z are X, Y, Z direction components of the magnetization of the S spin system; .gamma..sub.I, .gamma..sub.s are gyro-magnetic ratios of the spin systems I and S; J.sub.0 is the coupling constant between I spin system and S spin system; II is the vector of the spins I; is the vector of the spins S.
J.sub.R can be calculated according to this equation. In the calculation, since rf field for decoupling is divided into pulses in the pulse decoupling method as shown in FIG. 2, J.sub.R must be first calculated at each pulse by dividing the time t into pulses; thereafter, J.sub.R can be averaged. Then J.sub.R /J.sub.0 should be obtained.
In FIG. 4, a solid line represents a result of the calculation according to the aforesaid process under appropriate conditions. From this figure, it is known that there are two peaks in the power distribution between zero off-set and 3 KHz off-set frequency, beyond which decoupling becomes progressively more incomplete.